Analysis of the Convectively Modified GATE Boundary Layer Using in situand Acoustic Sounder Data

Abstract

A boundary-layer categorization scheme based on GATE acoustic sounder data stratifies surface data and tethered balloon data from the NOAA research ship Oceanographer. The results indicate a clear increase in the sensible heat flux across the sea surface during disturbed conditions (the gravity current and storm wake of the disturbance) but no conclusive differences of latent heat and momentum fluxes. The tethered balloon profiles show a near disappearance of the mixed layer within the gravity current and, in the storm wake, a very shallow and cool mixed layer capped by a strong stable layer relative to the undisturbed category. We calculate the vertical motion due to buoyancy-driven entrainment for a range on entrainment parameters after exploiting the categorized tethered balloon profiles to obtain mean gradients at the top of the mixed layer. Because we and others have observed that the shallow mixed-layer depth remains nearly constant with time in the storm wake, this calculated entrainment-induced vertical motion is balanced with an hypothesized mesoscale subsidence beneath the anvil in the wake. Even though our entrainment calculation ignores the possibly important but unknown effets of vertically propagating waves, breaking waves and wind shear, the integrated divergence derived from this subsidence agrees well with the range of mesoscale divergences in the storm wake presented by Zipser (1977).